How to write and read a file with a HashMap

Question

I have a HashMap with two Strings Map lt String  String gt  ldapContent   new HashMap lt String  String gt    Now I want to save the Map in an external file to use the Map later without initializing it again     So how must I save the Map to use it later again

User · Answer

You can write an object to a file using writeObject in ObjectOutputStream  ObjectOutputStream

User · Answer

The simplest solution that I can think of is using Properties class   Saving the map   Map lt String  String gt  ldapContent   new HashMap lt String  String gt     Properties properties   new Properties     for  Map Entry lt String String gt  entry   ldapContent entrySet          properties put entry getKey    entry getValue        properties store new FileOutputStream  data properties    null     Loading the map   Map lt String  String gt  ldapContent   new HashMap lt String  String gt     Properties properties   new Properties    properties load new FileInputStream  data properties      for  String key   properties stringPropertyNames         ldapContent put key  properties get key  toString         EDIT   if your map contains plaintext values  they will be visible if you open file data via any text editor  which is not the case if you serialize the map   ObjectOutputStream out   new ObjectOutputStream new FileOutputStream  data ser     out writeObject ldapContent   out close      EDIT2   instead of for loop  as suggested by OldCurmudgeon  in saving example   properties putAll ldapContent     however  for the loading example this is the best that can be done   ldapContent   new HashMap lt Object  Object gt  properties

User · Answer

since HashMap implements Serializable interface  you can simply use ObjectOutputStream class to write whole Map to file  and read it again using ObjectInputStream class     below simple code that explain usage of ObjectOutStream and ObjectInputStream  import java util    import java io    public class A       HashMap lt String String gt  hm      public A             hm new HashMap lt String String gt              hm put  1   A            hm put  2   B            hm put  3   C             method1 hm           public void method1 HashMap lt String String gt  map          write to file    fileone      try           File fileOne new File  fileone            FileOutputStream fos new FileOutputStream fileOne           ObjectOutputStream oos new ObjectOutputStream fos            oos writeObject map           oos flush            oos close            fos close          catch Exception e            read from file      try           File toRead new File  fileone            FileInputStream fis new FileInputStream toRead           ObjectInputStream ois new ObjectInputStream fis            HashMap lt String String gt  mapInFile  HashMap lt String String gt  ois readObject             ois close            fis close              print All data in MAP         for Map Entry lt String String gt  m  mapInFile entrySet                 System out println m getKey         m getValue                     catch Exception e          public static void main String args              new A           or if you want to write data as text to file you can simply iterate through Map and write key and value line by line  and read it again line by line and add to HashMap    import java util    import java io    public class A       HashMap lt String String gt  hm      public A            hm new HashMap lt String String gt              hm put  1   A            hm put  2   B            hm put  3   C             method2 hm           public void method2 HashMap lt String String gt  map          write to file    fileone      try           File fileTwo new File  filetwo txt            FileOutputStream fos new FileOutputStream fileTwo           PrintWriter pw new PrintWriter fos            for Map Entry lt String String gt  m  map entrySet                 pw println m getKey       m getValue                        pw flush            pw close            fos close          catch Exception e            read from file      try           File toRead new File  filetwo txt            FileInputStream fis new FileInputStream toRead            Scanner sc new Scanner fis            HashMap lt String String gt  mapInFile new HashMap lt String String gt                read data from file line by line          String currentLine          while sc hasNextLine                  currentLine sc nextLine                  now tokenize the currentLine              StringTokenizer st new StringTokenizer currentLine     false                 put tokens ot currentLine in map             mapInFile put st nextToken   st nextToken                       fis close               print All data in MAP         for Map Entry lt String String gt  m  mapInFile entrySet                  System out println m getKey         m getValue                    catch Exception e          public static void main String args              new A           NOTE  above code may not be the fastest way to doing this task  but i want to show some application of classes  See ObjectOutputStream   ObjectInputStream  HashMap  Serializable  StringTokenizer

User · Answer

HashMap implements Serializable so you can use normal serialization to write hashmap to file  Here is the link for Java - Serialization example

[java] How to write and read a file with a HashMap?

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